The present invention is directed to novel polymer dispersions and, more particularly, to polymer dispersions useful as replacements for casein in paper coatings, and to paper coating compositions containing the polymer dispersions.
Paper coating compositions for the preparation of coating printing papers normally consist of a slurry of pigments and fillers in an aqueous dispersion of a binder. The most commonly used binders are blends of natural binders, such as casein or starch, with synthetic binders which are usually polymer dispersions. Continuing attempts have been made and are going forward to replace casein in paper coatings, particularly for offset paper, with resin dispersions due to the high price; the irregularities and sensitivity to bacteria of casein. The primary requirement imposed on a resin dispersion to be used as a substitute for casein is a pigment binding capacity and wet rub resistance of the coated paper equal to that of, or obtained with casein without having the stability on shear stress and the desired low viscosity of the coating adversely influenced.
In the art, aqueous resin dispersions are described as the sole binder in paper coatings, i.e., no natural binders are used. For example, Deutsche Offenlegungsschrift No.2,123,857 describes mixed polymer dispersions thickened with alkali which are based on (meth)acrylates and other ethylenically unsaturated monomers, e.g., styrene, alkylene mono- or dicarboxylic acids and from 0 to 20 percent of monomers carrying one ethylenic bond, at least one polar group or additional ethylenic bonds. These dispersions are purported to convey enhanced water retention to the paper coatings either along or in admixture with other binder dispersions. However, these dispersions do not meet the criterion with respect to web rub resistance and, owing to the substantial thickening in the alkaline range, they cannot be used in satin white paper coatings. According to Deutsche Auslegeschrift No. 1,190,321, wet rub resistant papers are obtained using coatings containing an aqueous dispersion of acrylates, acrylic acid, and acrylamide, which are slightly alkaline due to the presence of volatile amines such as ammonia, ethylamine, and trimethylamine. These dispersions are incompatible in satin white paper coatings and, furthermore, they have a strong odor owing to the volatility of the amine. Austrian Pat. No. 253,349 describes a blend of dispersions A and B, dispersion A containing 90-10 percent by weight of vinyl acetate and/or vinyl propionate, 10-90 percent by weight of esters of acrylic acid and/or methacrylic acid with alcohols having from 4 to 8 carbon atoms, and 0-10 percent by weight of other copolymerizable ethylenically unsaturated compounds; and dispersion B containing 15-55 percent by weight of acrylic acid and/or methacrylic acid, 25-45 percent by weight of esters of acrylic acid and/or methacrylic acid with alcohols containing 1 to 4 carbon atoms, and 20-40 percent by weight of ethylenically unsaturated monomers which form water-insoluble homopolymers. Such a blend was found to be unusable in pigment combinations containing satin white. Furthermore, the claimed dispersions coated on paper do not have sufficient water resistance for coating on paper to be used for offset printing. A further disadvantage of the dispersions claimed in Austrian Pat. No. 253,349 is that they cannot be combined with other resin binders.